Monolithic microwave integrated circuits having both enhancement-mode and depletion mode transistors

ABSTRACT

A gallium nitride based monolithic microwave integrated circuit includes a substrate, a channel layer on the substrate and a barrier layer on the channel layer. A recess is provided in a top surface of the barrier layer. First gate, source and drain electrodes are provided on the barrier layer opposite the channel layer, with a bottom surface of the first gate electrode in direct contact with the barrier layer. Second gate, source and drain electrodes are also provided on the barrier layer opposite the channel layer. A gate insulating layer is provided in the recess in the barrier layer, and the second gate electrode is on the gate insulating layer opposite the barrier layer and extending into the recess. The first gate, source and drain electrodes comprise the electrodes of a depletion mode transistor, and the second gate, source and drain electrodes comprise the electrodes of an enhancement mode transistor.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 16/039,370, filed Jul. 19, 2018, the entire content of which isincorporated herein by reference.

STATEMENT OF U.S. GOVERNMENT INTEREST

This invention was made with Government support under Contract No.11-D-5309 awarded by the Department of Defense. The Government hascertain rights in the invention.

FIELD

The inventive concepts described herein relate to integrated circuitdevices and, more particularly, to monolithic microwave integratedcircuits.

BACKGROUND

Power semiconductor devices are widely used to carry large currents,support high voltages and/or operate at high frequencies such as radiofrequencies. A wide variety of power semiconductor devices are known inthe art including, for example, power switching devices and poweramplifiers. Many power semiconductor devices are implemented usingvarious types of Field Effect Transistors including, for example, HighElectron Mobility Transistors (HEMT) and Metal Oxide Semiconductor FieldEffect Transistors (MOSFETs).

Modern power semiconductor devices are generally fabricated from widebandgap semiconductor materials. For example, power HEMTs may befabricated from gallium arsenide (GaAs) based material systems or, morerecently, from gallium nitride (GaN) based material systems that areformed on a silicon carbide (SiC) substrate. Power semiconductor devicesmay be formed as discrete devices or as a plurality of devices (whichmay include transistors and other circuit devices such as resistors,inductors, capacitors, transmission lines and the like) that are formedon a common substrate to provide a so-called Monolithic MicrowaveIntegrated Circuit (MMIC). A MMIC refers to an integrated circuit thatoperates on radio and/or microwave frequency signals in which all of thecircuitry for a particular function is integrated into a singlesemiconductor chip. An example MMIC device is a transistor amplifierthat includes associated matching circuits, feed networks and the likethat are all implemented on a common substrate. MMIC transistoramplifiers typically include a plurality of unit cell HEMT transistorsthat are connected in parallel.

Field effect transistors such as HEMTs and MOSFETs may be classifiedinto depletion mode and enhancement mode types, corresponding to whetherthe transistor is in an ON-state or an OFF-state at a gate-sourcevoltage of zero. In enhancement mode devices, the devices are OFF atzero gate-source voltage, whereas in depletion mode devices, the deviceis ON at zero gate-source voltage. HEMTs are typically implemented asdepletion mode devices, in that they are conductive at a gate-sourcebias of zero due to the polarization-induced charge at the interface ofthe barrier and channel layers of the device.

SUMMARY

Pursuant to embodiments of the present invention, MMIC devices areprovided that include a monolithic substrate, a gallium nitride basedchannel layer on the monolithic substrate and a gallium nitride basedbarrier layer on the gallium nitride based channel layer opposite themonolithic substrate, the gallium nitride based barrier layer includinga recess in a top surface thereof. A first source electrode, a firstdrain electrode and a first gate electrode are provided on the galliumnitride based barrier layer opposite the gallium nitride based channellayer, the first gate electrode positioned between the first sourceelectrode and the first drain electrode with a bottom surface of thefirst gate electrode in direct contact with the gallium nitride basedbarrier layer. A second source electrode and a second drain electrodeare also provided on the gallium nitride based barrier layer oppositethe gallium nitride based channel layer. A gate insulating layer is inthe recess in the gallium nitride based barrier layer, and a second gateelectrode is on the gate insulating layer opposite the gallium nitridebased barrier layer, the second gate electrode positioned between thesecond source electrode and the second drain electrode and extendinginto the recess. The first source electrode, the first drain electrodeand the first gate electrode comprise electrodes of a depletion modetransistor, and the second source electrode, the second drain electrodeand the second gate electrode comprise electrodes of an enhancement modetransistor.

In some embodiments, the recess extends completely through the galliumnitride based barrier layer to expose the gallium nitride based channellayer. The recess may optionally further extend into a top surface ofthe gallium nitride based channel layer.

In some embodiments, the MMIC device may further include a third sourceelectrode, a third drain electrode and a third gate electrode on thegallium nitride based barrier layer opposite the gallium nitride basedchannel layer, the third gate electrode extending between the thirdsource electrode and the third drain electrode with a bottom surface ofthe third gate electrode in direct contact with the barrier layer. Insuch embodiments, the depletion mode transistor may be a first depletionmode transistor, and the third source electrode, the third drainelectrode and the third gate electrode may be the electrodes of a seconddepletion mode transistor.

In some embodiments, a first distance between the second sourceelectrode and the second gate electrode may be substantially the same asa second distance between the second drain electrode and the second gateelectrode. In some embodiments, a third distance between the firstsource electrode and the first gate electrode may be less than a fourthdistance between the first drain electrode and the first gate electrode.

In some embodiments, the MMIC device may further include an insulatinglayer on the gallium nitride based barrier layer that has openings foreach of the first and second source electrodes, the first and seconddrain electrodes and the first and second gate electrodes. In suchembodiments, the insulating layer and the gate insulating layer may beformed of different materials, and the gate insulating layer may extendalong a top surface of at least a portion of the insulating layer andalong sidewalls of the opening in the insulating layer for the secondgate electrode.

In some embodiments, the gate insulating layer may be an oxide layer andthe insulating layer may be a nitride layer.

In some embodiments, the depletion mode transistor may include a fieldplate and the enhancement mode transistor does not include a fieldplate.

Pursuant to further embodiments of the present invention, semiconductorintegrated circuits are provided that include a substrate, a radiofrequency (RF) power amplifier formed on a first region of thesubstrate, the RF power amplifier including a plurality of galliumnitride based depletion mode transistors and a digital circuit formed ona second region of the substrate, the digital circuit including aplurality of gallium nitride based enhancement mode transistors.

In some embodiments, the digital circuit may further include a pluralityof gallium nitride based depletion mode transistors.

In some embodiments, the semiconductor integrated circuit may furtherinclude a gallium nitride based channel layer on the substrate and agallium nitride based barrier layer on the gallium nitride based channellayer opposite the substrate, the gallium nitride based barrier layerincluding a plurality of recesses in a top surface thereof. The gateelectrodes of the gallium nitride based depletion mode transistors inthe first region of the substrate may directly contact the galliumnitride based barrier layer, and the gate electrodes of the galliumnitride based enhancement mode transistors in the second region of thesubstrate may extend into the respective recesses in the gallium nitridebased barrier layer.

In some embodiments, the semiconductor integrated circuit may furtherinclude a gate insulating layer in the recesses in the gallium nitridebased barrier layer between the gallium nitride based barrier layer andthe respective gate electrodes of the gallium nitride based enhancementmode transistors.

In some embodiments, the recesses extend completely through the galliumnitride based barrier layer to expose the gallium nitride based channellayer.

In some embodiments, the recesses further extend into a top surface ofthe gallium nitride based channel layer.

In some embodiments, the gate insulating layer comprises an oxide layer.

In some embodiments, the recesses only extend part of the way throughthe gallium nitride based barrier layer, and wherein the gate electrodesof the gallium nitride based enhancement mode transistors in the secondregion of the substrate directly contact respective portions of thegallium nitride based barrier layer.

In some embodiments, each gallium nitride based enhancement modetransistor includes a gate electrode, a source electrode and a drainelectrode, and wherein the gate electrode of each gallium nitride basedenhancement mode transistor is equidistant between its correspondingsource and drain electrodes.

In some embodiments, each gallium nitride based depletion modetransistor includes a gate electrode, a source electrode and a drainelectrode, and wherein the gate electrode of each gallium nitride baseddepletion mode transistor is closer to its corresponding sourceelectrode than it is to its corresponding drain electrode.

In some embodiments, the semiconductor integrated circuit may furtherinclude a gallium nitride based channel layer on the substrate and agallium nitride based barrier layer on the gallium nitride based channellayer opposite the substrate, where each gallium nitride basedenhancement mode transistor includes a gate electrode, a sourceelectrode and a drain electrode, and the gallium nitride based barrierlayer is doped with first conductivity type dopants under the source anddrain electrodes of each gallium nitride based enhancement modetransistor, and the gallium nitride based barrier layer is doped withsecond conductivity type dopants under the gate electrodes of eachgallium nitride based enhancement mode transistor, wherein the firstconductivity type is opposite the second conductivity type.

Pursuant to yet further embodiments of the present invention,semiconductor integrated circuits are provided that include a monolithicsubstrate, a first gallium nitride based depletion mode transistor on afirst region of the monolithic substrate, the first gallium nitridebased depletion mode transistor having a first gate width and a firstgate length, a second gallium nitride based depletion mode transistor ona second region of the monolithic substrate, the second gallium nitridebased depletion mode transistor having a second gate width and a secondgate length and a gallium nitride based enhancement mode transistor onthe second region of the monolithic substrate, the gallium nitride basedenhancement mode transistor having a third gate width and a third gatelength.

In some embodiments, the first gate width exceeds the second gate widthby at least a factor of ten.

In some embodiments, the first gate length is less than the second gatelength.

In some embodiments, the first gate width exceeds the third gate widthby at least a factor of ten.

In some embodiments, the first gate length is less than the third gatelength.

In some embodiments, the second gate length exceeds the third gatelength.

In some embodiments, the semiconductor integrated circuit may furtherinclude a gallium nitride based channel layer on the monolithicsubstrate and a gallium nitride based barrier layer on the galliumnitride based channel layer opposite the monolithic substrate, a topsurface of the gallium nitride based barrier layer including a recess.In such embodiments, a gate electrode of the first gallium nitride baseddepletion mode transistor and a gate electrode of the second galliumnitride based depletion mode transistor may each directly contact thegallium nitride based barrier layer, and a gate electrode of the galliumnitride based enhancement mode transistor extends into the recess in thegallium nitride based barrier layer.

In some embodiments, the semiconductor integrated circuit may furtherinclude a gate insulating layer in the recess in the gallium nitridebased barrier layer between the gallium nitride based barrier layer andthe gate electrode of the gallium nitride based enhancement modetransistor.

In some embodiments, the recess extends completely through the galliumnitride based barrier layer to expose the gallium nitride based channellayer.

In some embodiments, the recess further extends into a top surface ofthe gallium nitride based channel layer.

In some embodiments, the gate insulating layer comprises an oxide layer.

In some embodiments, the recess only extends part of the way through thegallium nitride based barrier layer, and wherein the gate electrode ofthe gallium nitride based enhancement mode transistor directly contactssidewalls and a bottom surface of the recess.

In some embodiments, the semiconductor integrated circuit may furtherinclude a gallium nitride based channel layer on the monolithicsemiconductor substrate and a gallium nitride based barrier layer on thegallium nitride based channel layer opposite the monolithicsemiconductor substrate. In such embodiments, the gallium nitride basedenhancement mode transistor may include a gate electrode, a sourceelectrode and a drain electrode and the gallium nitride based barrierlayer may be doped with first conductivity type dopants under the sourceand drain electrodes of the gallium nitride based enhancement modetransistor, and the gallium nitride based barrier layer may be dopedwith second conductivity type dopants under the gate electrode of thegallium nitride based enhancement mode transistor, where the firstconductivity type is opposite the second conductivity type.

Pursuant to additional embodiments of the present invention, methods offabricating a gallium nitride based monolithic microwave integratedcircuit device are provided in which a gallium nitride based channellayer is formed on a substrate. A gallium nitride based barrier layer isformed on the gallium nitride based channel layer opposite thesubstrate. An insulating layer is formed on the gallium nitride basedbarrier layer, the insulating layer including a plurality of first gateelectrode openings that expose the gallium nitride based barrier layer,a plurality of second gate electrode openings that expose the galliumnitride based barrier layer and a plurality of source/drain electrodeopenings that expose the gallium nitride based barrier layer. Recessesare formed in respective portions of the gallium nitride based barrierlayer that are exposed by the first gate electrode openings. A gateinsulating layer is formed in the first gate electrode openings, thegate insulating layer covering sidewalls and bottom surfaces of therespective recesses. A plurality of first source electrodes, a pluralityof second source electrodes, a plurality of first drain electrodes and aplurality of second drain electrodes are formed in the source/drainelectrode openings in the insulating layer, the first source electrodes,the second source electrodes, the first drain electrodes and the seconddrain electrodes directly contacting a top surface of the galliumnitride based barrier layer. First gate electrodes are formed in thefirst gate electrode openings on the gate insulating layer, each firstgate electrode extending into a respective one of the recesses. Secondgate electrodes are formed in second gate electrode openings in theinsulating layer, the second gate electrodes directly contacting a topsurface of the gallium nitride based barrier layer. Each set of a firstsource electrode, a first drain electrode and a first gate electrodecomprises the electrodes of an enhancement mode transistor, and each setof a second source electrode, a second drain electrode and a second gateelectrode comprises the electrodes of a depletion mode transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of a conventional gallium nitride basedpower semiconductor device that is controlled using digital controlsignals supplied from external control circuit.

FIG. 2A is a schematic cross-sectional view of three regions of agallium nitride based MMIC device according to embodiments of thepresent invention that includes both depletion-mode and enhancement modetransistors.

FIGS. 2B-2D are schematic plan views corresponding to the threecross-sectional regions illustrated in FIG. 2A.

FIG. 3 is a cross-sectional view of an alternative enhancement modetransistor design that may be used in gallium nitride based MMIC devicesaccording to embodiments of the present invention.

FIG. 4 is a cross-sectional view of another alternative enhancement modetransistor design that may be used in gallium nitride based MMIC devicesaccording to embodiments of the present invention.

FIGS. 5A-5C are schematic plan views of gallium nitride based MMICdevices according to various embodiments of the present invention.

FIG. 6 is a flow chart describing a method of fabricating a galliumnitride based MMIC device according to certain embodiments of thepresent invention.

DETAILED DESCRIPTION

Gallium nitride based power semiconductor devices such as galliumnitride based HEMT devices are very promising candidates for high powerRF applications such as high power amplifiers used in radiocommunications systems, radar and various other wireless applications.As used herein, the term “gallium nitride based” refers to thosesemiconducting compounds that include at least gallium and nitrogen,including gallium nitride as well as ternary and quaternary compoundssuch as aluminum gallium nitride (AlGaN) and aluminum indium galliumnitride (AlInGaN). Gallium nitride based power semiconductor deviceshave been developed as both discrete devices that are coupled with othercircuitry such as, for example, impedance matching networks, or as MMICdevices (e.g., a multi-stage HEMT amplifier with built-in impedancematching networks). In many applications, the gallium nitride basedpower semiconductor devices are controlled by control signals.Typically, commercially available digital circuits are used to generatethe control signals that are supplied to a gallium nitride based powersemiconductor device. These digital circuits may comprise one or moreadditional semiconductor chips, and are typically silicon-basedsemiconductor devices.

FIG. 1 is a schematic plan view of a conventional gallium nitride basedpower semiconductor device that is controlled using digital controlsignals. As shown in FIG. 1, a gallium nitride based integrated circuitchip 10 is provided. The gallium nitride based integrated circuit chip10 may include a base substrate 12 such as, for example, a siliconcarbide semiconductor substrate that has a plurality of gallium nitridebased epitaxial layers formed thereon. Various input/output pads may beformed on the substrate 12 including, for example, RF input pads 14, RFoutput pads 16, bias signal pads 18 and control signal pads 20. Acontrol circuit integrated circuit chip 30 may also be provided. Thecontrol circuit integrated circuit chip 30 may be implemented on asecond semiconductor substrate 32, which may comprise, for example, asilicon substrate. Various input/output pads may be formed on thesubstrate 32 of the control circuit integrated circuit chip 30including, for example, bias signal pads 34, control signal input pads36 and control signal output pads 38. While FIG. 1 illustrates a singlecontrol circuit integrated circuit chip 30, it will be appreciated thata plurality of separate control circuit integrated circuit chips 30 maybe needed to generate the necessary control signals, particularly ifoff-the-shelf digital circuits are used to form the control circuitry.

In some cases (such as the example of FIG. 1), the gallium nitride basedintegrated circuit chip 10 and the control circuit integrated circuitchip 30 may be mounted in a single package. As shown in FIG. 1, in suchembodiments, a plurality of bond wires or other interconnections 40 maybe provided that connect various output pads 38 included in controlcircuit integrated circuit chip 30 to input pads 20 included in thegallium nitride based integrated circuit chip 10. In other cases, thetwo integrated circuit chips 10, 30 may be packaged separately andinterconnected, for example, via control lines on a printed circuitboard and/or bond wires. In either case, interconnecting the twointegrated circuit chips 10, 30 tends to add significant additionalcomplexity and expense to the fabrication process, and also may reducethe performance of the device.

Pursuant to embodiments of the present invention, gallium nitride basedMMIC devices are provided in which both an RF circuit and the controlcircuits that are used to generate digital control signals that controlthe operation of the RF circuit are formed on a single monolithicsubstrate. Typically, gallium nitride based RF circuits are formed usingdepletion mode (normally on) transistors. However, digital controlcircuits that are formed exclusively using depletion mode transistorsare more difficult to design, require significantly more chip area toimplement, and use more power as compared to digital control circuitsthat are implemented using a combination of enhancement mode anddepletion mode transistors, or digital control circuits that areimplemented exclusively using enhancement mode transistors.Consequently, digital control circuits have not been integrated intoconventional gallium nitride based RF MMIC devices.

The gallium nitride based MMIC devices according to embodiments of thepresent invention may be smaller, cheaper and less complex as comparedto conventional multi-chip circuits that provide the same functionality.Moreover, various performance advantages may be achieved by integratingthe digital control circuitry into the gallium nitride based MMIC,including improved high temperature performance and faster operatingspeeds. Moreover, the gallium nitride based MMIC devices according toembodiments of the present invention may be fabricated using currentlyavailable tools and fabrication techniques.

In some embodiments, gallium nitride based MMIC devices are providedthat include a gallium nitride based epitaxial structure that is formedon a substrate such as, for example, a silicon carbide substrate. Thegallium nitride based epitaxial structure may include, for example, agallium nitride based channel layer and a gallium nitride based barrierlayer that is formed on the gallium nitride based channel layer oppositethe substrate. Additional epitaxial layers may be included as part ofeither the gallium nitride base channel layer or the gallium nitridebased barrier layer such as, for example, buffer layers, strainbalancing layers, transition layers and the like. A plurality ofdepletion mode transistors may be formed on and in the gallium nitridebased epitaxial structure. These depletion mode transistors may includea gate electrode, a source electrode and a drain electrode. Theseelectrodes may be formed on the gallium nitride based barrier layeropposite the gallium nitride based channel layer and may be in directcontact with the gallium nitride based barrier layer.

Additionally, a plurality of enhancement mode transistors may be formedon and in the gallium nitride based epitaxial structure. The enhancementmode transistors may likewise include a gate electrode, a sourceelectrode and a drain electrode. The source and drain electrodes may beformed on the gallium nitride based barrier layer and may be in directcontact with the gallium nitride based barrier layer. A portion of thegallium nitride based barrier layer that is between the source and drainelectrodes may be etched away to form an opening in the barrier layerthat exposes the channel layer. A gate insulating layer such as, forexample, an oxide layer (e.g., SiO₂) may be formed in the opening, andthe gate electrode may be formed on the gate insulating layer oppositethe gallium nitride based channel layer and the gallium nitride basedbarrier layer.

In other embodiments, the enhancement mode transistors may have adifferent design in which the portion of the gallium nitride basedbarrier layer that is between the source and drain electrodes may onlybe partially etched away so that the opening formed in the galliumnitride based barrier layer does not expose the gallium nitride basedchannel layer. The gate electrode is formed in the opening in thegallium nitride based barrier layer.

In some embodiments, the depletion mode transistors may comprise “RF”depletion mode transistors that are configured to operate on radiofrequency (“RF”) signals, such as the transistors of a gallium nitridebased HEMT RF amplifier. In some embodiments, the depletion modetransistors may additionally include “digital” depletion modetransistors that are part of a digital control circuit. The size andlayout of the digital depletion mode transistors and the RF depletionmode transistors may be different. The enhancement mode transistors maybe “digital” enhancement mode transistors that are part of the digitalcontrol circuit.

Embodiments of the present invention will now be described in greaterdetail with reference to FIGS. 2A-6.

FIG. 2A is a schematic cross-sectional view of three regions of agallium nitride based MMIC device according to embodiments of thepresent invention that includes both depletion-mode and enhancement modetransistors. FIGS. 2B-2D are schematic plan views corresponding to thethree cross-sectional regions illustrated in FIG. 2A.

Referring FIGS. 2A-2D, it can be seen that a gallium nitride based MMICdevice 100 includes three different types of transistors, namely RFdepletion mode transistors 200, digital depletion mode transistors 300and digital enhancement mode transistors 400. While FIG. 2A onlyillustrates a single instance of each transistor 200, 300, 400 tosimplify the drawing, it will be appreciated that typically the galliumnitride based MMIC device 100 will include multiple of each type oftransistor 200, 300, 400. It will also be appreciated that the digitaldepletion mode transistor 300 may be omitted in some embodiments.

As shown in FIG. 2A, the RF depletion mode transistors 200, digitaldepletion mode transistors 300 and digital enhancement mode transistors400 may all be formed on a monolithic substrate 110 such as, forexample, a silicon carbide semiconductor substrate 110. For example, thesubstrate 110 may comprise a or 6H-SiC substrate. Silicon carbide has amuch closer crystal lattice match to gallium nitride based materialsthan does sapphire (Al₂O₃), which is a very common substrate materialfor gallium nitride based devices. The closer lattice match of siliconcarbide may result in gallium nitride based materials of higher qualitythan those generally available on sapphire. Silicon carbide also has avery high thermal conductivity so that the total output power of galliumnitride based devices on silicon carbide is, typically, not as limitedby thermal dissipation of the substrate as in the case of the samedevices formed on sapphire. Also, the availability of semi-insulatingsilicon carbide substrates may provide for device isolation and reducedparasitic capacitance. Although silicon carbide may be used as asubstrate material, embodiments of the present invention may utilize anysuitable substrate, such as sapphire, aluminum nitride, aluminum galliumnitride, gallium nitride, silicon, GaAs, LGO, ZnO, LAO, InP and thelike.

As shown in FIG. 2A, a gallium nitride based epitaxial structure 120 isformed on the substrate 110. The gallium nitride based epitaxialstructure 120 may include a first group of one or more gallium nitridebased layers 130. The first group of gallium nitride based layers 130may include a gallium nitride based channel layer, such asAl_(x)Ga_(1-x)N, where 0≤x<1. In certain embodiments of the presentinvention, the gallium nitride based channel layer may have the formulaAl_(x)Ga_(1-x)N, where x=0, indicating that the channel layer is agallium nitride layer. However, it will be appreciated that in otherembodiments the gallium nitride based channel layer may also be adifferent gallium nitride based layer such as InGaN, AlInGaN or thelike. In an example embodiment, the gallium nitride based channel layermay be undoped or unintentionally doped and may be grown to a thicknessof greater than about 20 Å. The gallium nitride based channel layer mayalso be a multi-layer structure, such as a superlattice, and may includecombinations of GaN, AlGaN and the like. In some embodiments, the firstgroup of gallium nitride based layers 130 may include a gallium nitridebased channel layer and one or more additional layers such as, forexample buffer, nucleation, transition and/or strain balancing layers.For example, an aluminum nitride buffer layer may be included to providean appropriate crystal structure transition between the silicon carbidesubstrate and the remainder of the device. Herein, the first group ofgallium nitride based layers 130 will be referred to as the galliumnitride based channel layer 130 regardless as to whether or not thefirst group of gallium nitride based layers only includes a galliumnitride based channel layer or also includes additional layers.

The gallium nitride based epitaxial structure 120 may further include asecond group of one or more gallium nitride based layers 140. The secondgroup of gallium nitride based layers 140 may include a gallium nitridebased barrier layer, and may also include additional layers such astransition and/or strain balancing layers. Herein, the second group ofgallium nitride based layers will be referred to as the gallium nitridebased barrier layer 140 regardless as to whether or not the second groupof gallium nitride based layers only includes a gallium nitride basedbarrier layer or also includes additional layers. A bandgap of a lowerportion of the gallium nitride based barrier layer 140 that contacts theupper surface of the gallium nitride based channel layer 130 may exceedthe bandgap of the uppermost layer of the gallium nitride based channellayer 130. Additionally, the gallium nitride based channel layer 130 mayhave a larger electron affinity than the gallium nitride based barrierlayer 140. The energy of the conduction band edge of the gallium nitridebased channel layer 130 is less than the energy of the conduction bandedge of the gallium nitride based barrier layer 140 at the interfacebetween the gallium nitride based channel and barrier layers 130, 140.

In certain embodiments, the gallium nitride based barrier layer 140 isAlN, AlInN, AlGaN or AlInGaN, or combinations of layers thereof, with athickness of between about 0.1 nm and about 30 nm. In some embodimentsof the present invention, the gallium nitride based barrier layer 140 isAl_(x)Ga_(1-x)N where 0<x<1. In particular embodiments, the aluminumconcentration is about 25%. However, in other embodiments of the presentinvention, the gallium nitride based barrier layer 140 comprises AlGaNwith an aluminum concentration of between about 5% and about 100%. Inspecific embodiments of the present invention, the aluminumconcentration is greater than about 10%. In some embodiments, thegallium nitride based barrier layer 140 may be undoped or doped with ann-type dopant to a concentration less than about 10¹⁹ cm⁻³. The galliumnitride based barrier layer 140 may be thick enough and have a highenough aluminum concentration to induce a significant carrierconcentration at the interface between the gallium nitride based channellayer 130 and the gallium nitride based barrier layer 140. In an exampleembodiment, the uppermost portion of the gallium nitride based channellayer 130 may comprise gallium nitride, while the lowermost portion ofthe gallium nitride based barrier layer 140 may comprise aluminumgallium nitride.

As is further shown in FIG. 2A, the substrate 110 may include a firstregion 112 and a second region 114. The RF depletion mode transistors200 may be formed in the first region 112 of the substrate 110, and thedigital depletion mode transistors 300 and the digital enhancement modetransistors 400 may be formed in the second region 114 of the substrate110.

Referring now to FIGS. 2A and 2B, each RF depletion mode transistor 200includes a gate electrode 210, a source electrode 220 and a drainelectrode 230. The gate electrode 210 may comprise a relatively narrowgate finger 210 that extends in parallel between the source electrode220 and the drain electrode 230. The “gate width” of a transistor refersto the distance by which the gate finger 210 extends between or“overlaps” with its associated source and drain electrodes 220, 230. Asshown in FIG. 2B, the gate width of the RF depletion mode transistors200 may be relatively large. In the depicted embodiment, the width ofthe gate finger 210 of the RF depletion mode transistor is about 500microns.

As is further shown in FIG. 2A, source/drain regions 142 may be formedin the gallium nitride based barrier layer 140 underneath the respectivesource and drain electrodes 220, 230. The source/drain regions 142 mayextend into the gallium nitride based channel layer 130, as shown. Thesource/drain regions 142 may be doped (or more heavily doped) regions ofthe gallium nitride based barrier layer 140. The source/drain regions142 may be formed, for example, by ion implantation. The source/drainregions 142 may have any suitable doping concentration. In an exampleembodiment, the source/drain regions 142 may have a doping concentrationof about 1×10²⁰ to 1×10²¹ cm³. In other embodiments, the source/drainregions 142 may only extend part of the way rather than completelythrough the gallium nitride based barrier layer 140. In still otherembodiments, the source/drain regions 142 may be omitted so that thesource and drain electrodes 220, 230 are formed directly on the galliumnitride based barrier layer 140. The digital depletion mode transistors300 and the digital enhancement mode transistors 400 that are discussedin more detail below may have any of the above-discussed configurationsfor the source/drain regions 142, including omitting the source/drainregions 142.

While not shown in the figures, it will be understood that a pluralityof RF depletion mode transistors 200 may be provided in the first region112 of the substrate 110. These RF depletion mode transistors 200 maycomprise a plurality of unit cell transistors that are electricallyconnected to each other in parallel. Each unit cell transistor 200 mayshare a source electrode 220 and/or a drain electrode 230 with one ormore adjacent unit cell transistors 200.

An insulating layer 150 is formed on a top surface of the galliumnitride based barrier layer 140. The insulating layer 150 may comprise,for example, a nitride layer such as a silicon nitride layer. Theinsulating layer 150 may serve as a passivation layer in someembodiments. The insulating layer 150 may additionally (oralternatively) insulate wings 212 that extend laterally from the upperportion of the gate electrode 210 from the gallium nitride based barrierlayer 140. Openings are provided in the insulating layer 150 for therespective gate, source and drain electrodes 210, 220 x, 230. Theseopenings expose the gallium nitride based barrier layer 140 so that thegate, source and drain electrodes 210, 220, 230 may directly contact atop surface of the gallium nitride based barrier layer 140.

The depletion mode transistors 200 may comprise HEMT transistors. Due tothe difference in bandgap between the gallium nitride based barrierlayer 140 and the gallium nitride based channel layer 130 andpiezoelectric effects at the interface between the gallium nitride basedbarrier layer 140 and the gallium nitride based channel layer 130, a twodimensional electron gas (2DEG) is induced in the gallium nitride basedchannel layer 130 at a junction between the gallium nitride basedchannel layer 130 and the gallium nitride based barrier layer 140. The2DEG acts as a highly conductive layer that allows conduction betweensource and drain regions of the depletion mode transistor 200 that arebeneath the source electrode 220 and the drain electrode 230,respectively. The source electrode 220 and the drain electrode 230 maydirectly contact the barrier layer 140. The gate fingers 210 also maydirectly contact the gallium nitride based barrier layer 140 and arepositioned between the source and drain electrodes 220, 230. While thegate fingers 210 and source and drain electrodes 220, 230 are all shownin FIG. 2A as having the same “length” (i.e., the distance that theelectrodes extend in the X direction), it will be appreciated that inpractice the gate fingers 210 have lengths that are substantiallysmaller than the lengths of the source and drain electrodes 220, 230, ascan be seen in FIG. 2B. It will also be appreciated that the source anddrain electrodes 220, 230 need not have the same lengths.

The gate finger 210 may comprise a metal gate finger in someembodiments. The particular material(s) used to form the gate finger 210may be chosen based on, for example, the composition of the galliumnitride based barrier layer 140. In example embodiments, the gate finger210 may comprise one or more of Ni, Pt and Au. The source and drainelectrodes 220, 230 may include one or more metals such as Ti, Ni andPt. In an example embodiment, the source and drain electrodes 220, 230may comprise a Ti/Si/Ni/Pt stack. The source and drain electrodes 220,230 may form ohmic contacts to the gallium nitride based barrier layer140. The gate electrode 210 may be closer to the source electrode 220than it is to the drain electrode 230, as is shown in FIG. 2B.

As is further shown in FIGS. 2A and 2B, a spacer layer 240 may be formedon top of the gate electrode 210 and on top portions of the insulatinglayer 150. A field plate 250 may be formed on top of the spacer layer240. The field plate 250 may vertically overlap both a portion of thegate electrode 210 and a portion of the semiconductor structure 120 thatis between the gate electrode 210 and the drain electrode 230. The fieldplate 250 is electrically isolated from both the gate electrode 210 andportion of the semiconductor structure 120 that the field plate 250vertically overlaps. The field plate 250 may be connected to the sourceelectrode 220 via an electrical connection that is not shown in thefigures. The field plate 250 is a conductive plate that redistributesthe electric field on the drain side of the transistor 200 in order toimprove the breakdown voltage, gain, and maximum operating frequency ofthe device.

FIGS. 2A and 2C illustrate the design of the digital depletion modetransistor 300. The digital depletion mode transistor 300 may havesimilar layers and regions as the RF depletion mode transistor 200, butthe sizes of the various regions may be quite different. As shown inFIGS. 2A and 2C, the digital depletion mode transistor 300 includes agallium nitride based channel layer 130 that is formed on a substrate110, and a gallium nitride based barrier layer 140 that is on thegallium nitride based channel layer 130 opposite the substrate 110. Thegallium nitride based barrier layer 140 may include source/drain regions142. A gate electrode 310, a source electrode 320 and a drain electrode330 are formed on the gallium nitride based barrier layer 140. The gateelectrode 310 extends in parallel between the source electrode 320 andthe drain electrode 330.

The above discussed insulating layer 150 (e.g., a silicon nitrideinsulating layer) is also formed on a top surface of the gallium nitridebased barrier layer 140 in the second region 114 of the substrate 110.The gate, source and drain electrodes 310, 320, 330 are formed inrespective openings in the insulating layer 150. The depletion modetransistors 300 may, like the depletion mode transistor 200, comprise aHEMT transistor and may operate in the same fashion as the depletionmode transistor 200 discussed above.

As shown in FIG. 2C, the gate width of the digital depletion modetransistor 300 may be orders of magnitude smaller than the gate width ofthe RF depletion mode transistor 200. In the depicted embodiment, thewidth of the gate electrode 310 of the digital depletion mode transistoris about 3 microns, compared to a gate width of about 500 microns forthe RF depletion mode transistor 200. In the depicted embodiment, thelength of the gate electrode 310 of the digital depletion modetransistor is about 6-7 microns, compared to a gate length of about0.1-0.5 microns for the RF depletion mode transistor 200. As is alsoshown in FIG. 2C, the gate width of each of the depletion modetransistors 300 may be about half the gate length thereof in an exampleembodiment. The gate, source and drain electrodes 310, 320, 330 may beformed of the same materials as the corresponding materials used to formthe gate, source and drain electrodes 210, 220, 230 of depletion modetransistor 200. The field plates 250 that are included in the depletionmode transistors 200 may be omitted in the depletion mode transistors300. Additionally, the gate electrode 310 may be located substantiallyequidistant from the source electrode 320 and the drain electrode 330 inthe depletion mode transistors 300.

Referring next to FIGS. 2A and 2D, each digital enhancement modetransistor 400 includes a gate electrode 410 that is positioned betweena source electrode 420 and a drain electrode 430. The gate width of thedigital enhancement mode transistor 400 may be several orders ofmagnitude smaller than the gate width of the RF depletion modetransistors 200, and may be about the same as the gate width of thedigital depletion mode transistors 300. The gate length of the digitalenhancement mode transistor 400 may be an order of magnitude smallerthan the gate length of the digital depletion mode transistor 300.

The digital enhancement mode transistor 400 may be similar to thedigital depletion mode transistor 300 described above, but may have adifferent gate electrode design. In particular, referring first to FIG.2A, it can be seen that a recess 412 is formed in the gallium nitridebased barrier layer 140, and the gate electrode 410 extends into therecess 412. The recess 412 may extend completely through the galliumnitride based barrier layer 140, and may extend into the gallium nitridebased channel layer 130 in some embodiments. For example, in someembodiments, the recess 412 may extend between 1 nm and 15 nm into thegallium nitride based channel layer 130. A gate insulating layer 414 isformed in the recess 412. The gate insulating layer 414 may cover theexposed sidewalls of the gallium nitride based barrier layer 140 as wellas the exposed top surface of the gallium nitride based channel layer130 in order to isolate the gate electrode 410 from the gallium nitridebased channel layer 130. The modified gate electrode configurationcauses the transistor 400 to operate as an enhancement mode device.Advantageously, the enhancement mode transistor 400 may be formed usingthe same general processing steps used to form the depletion modetransistors 200, 300, allowing both depletion mode and enhancement modetransistors to be readily formed on the same substrate.

The digital enhancement mode transistor 400 may have a gate width thatis similar to the gate width of the digital depletion mode transistor300 in some embodiments. For example, the gate width of each digitalenhancement mode transistor 400 may be about 2-6 microns. The gatelength of each digital enhancement mode transistor 400 may be smallerthan the gate lengths of the digital depletion mode transistors. Forexample, the gate length of each digital enhancement mode transistor 400may be less than 1 micron.

The RF depletion mode transistors 200 are typically implemented as “unitcell” transistors, where a plurality of individual “unit cell”transistors are formed that are electrically connected in parallel so asto operate as a single transistor. Many unit cells may be provided toincrease the current carrying capacity and voltage blocking capabilitiesof the device. In contrast, the digital depletion mode transistors 300and the digital enhancement mode transistors 400 are typicallyimplemented as stand-alone transistors since these transistors pass muchsmaller current levels.

In some embodiments, the RF depletion mode transistors 200 may comprisean RF power amplifier, and the digital depletion mode transistors 300and the digital enhancement mode transistors 400 may be arranged to formdigital control circuits that gate the RF signal that is input to the RFpower amplifier. The digital depletion mode transistors 300 and thedigital enhancement mode transistors 400 may be low voltage devices thatgenerate relatively low electric fields.

The gallium nitride based MMIC devices according to embodiments of thepresent invention may exhibit a number of advantages. For example, RFtransistors and digital control transistors may be formed on a commonsubstrate, allowing transmission lines that connect the digital controlcircuits to the RF circuit to be formed on the common substrate. Thismay avoid the need, and the associated cost, for interconnectingmultiple chips via wire bonding or other techniques. As such, thegallium nitride based MMIC devices according to embodiments of thepresent invention may be smaller, cheaper and less complex as comparedto conventional multi-chip circuits that provide the same functionality.Moreover, the devices according to embodiments of the present inventionmay have much shorter control lines and hence may exhibit improvedperformance. Additionally, by fabricating the digital transistors 300,400 as gallium nitride based transistors the high temperatureperformance and over all robustness of the circuit may be improved.Moreover, the digital enhancement mode transistors 400 may be formedusing substantially the same process steps that are used to form thedepletion mode transistors 200, 300, allowing the same processingequipment to be used to form all three types of transistors on thecommon substrate. Fabrication of the enhancement mode transistors 400may require the additional steps of forming the recess 412 in thegallium nitride based barrier layer 140 and forming the gate insulatinglayer 414 in the second region of the substrate 110, but these are bothstandard processing steps that may readily be incorporated into themanufacturing process.

The gallium nitride based enhancement mode transistor as shown in FIGS.2A and 2D has a non-traditional design that may not represent the idealway to fabricate a gallium nitride based enhancement mode transistor.However, this design exhibits certain advantages when used to form agallium nitride based MMIC device that has integrated control circuitry,as the enhancement mode transistor may readily be fabricated during theprocessing steps used to form the RF transistors.

Pursuant to further embodiments of the present invention, galliumnitride based MMIC devices may be provided that include enhancement modetransistors that have alternative designs. FIGS. 3 and 4 illustrate twoalternate enhancement mode transistor designs that may be used ingallium nitride based MMIC devices according to embodiments of thepresent invention. The enhancement mode transistor designs illustratedin FIGS. 3 and 4 may be used in place of the enhancement mode transistor400 of FIG. 2A.

FIG. 3 is a cross-sectional view of an alternative enhancement modetransistor 500 that may be used in gallium nitride based MMIC devicesaccording to embodiments of the present invention. As can be seen bycomparing FIG. 3 to FIG. 2A, the enhancement mode transistor 500 may besimilar to the enhancement mode transistor 400 that is described above,but the enhancement mode transistor 500 includes a recess 512 that doesnot extend completely through the gallium nitride based barrier layer140. The gate electrode 510 is formed in the recess 512 to cover thesidewalls of the recess 512 and the bottom surface of the recess 512.

In the enhancement mode transistor 500, a gate insulating layer 414 mayoptionally be provided, as it may increase the forward gate voltage ofthe device. However, it will be appreciated that the gate insulatinglayer 414 may be omitted in other embodiments as the remaining portionof the gallium nitride based barrier layer 140 may prevent the gateelectrode 510 from forming a short circuit with the gallium nitridebased channel layer 130. In embodiments that include the gate insulatinglayer 414, the gate insulating layer 414 may be a high dielectricconstant material (i.e., a material having a dielectric constant that ishigher than the dielectric constant of silicon oxide) such as, forexample, aluminum oxide, halfnium oxide, zirconium oxide or any otherappropriate high dielectric constant material. The gate electrode 510may include nickel oxide in some embodiments, which may advantageouslyincrease the threshold voltage of the enhancement mode transistor 500.

FIG. 4 is a cross-sectional view of another alternative enhancement modetransistor 600 that may be used in gallium nitride based MMIC devicesaccording to still further embodiments of the present invention. Asshown in FIG. 6, the enhancement mode transistor 600 has a somewhatdifferent design than the enhancement mode transistor 400 of FIGS. 2Aand 2D. In particular, the enhancement mode transistor 600 does notinclude a recess in the gallium nitride barrier layer 640, and also doesnot include any gate insulating layer. The gallium nitride based barrierlayer 640 of enhancement mode transistor 600, however, does include adoped region 660 underneath the gate electrode 610 that is doped withsecond conductivity type dopants. For example, if the source/drainregions 142 in the gallium nitride based barrier layer 640 are dopedwith n-type dopants, then the doped region 660 may be doped with p-typedopants. The doped region 660 may be formed, for example, by ionimplantation. In an example embodiment, the doped region 660 may bedoped with p-type dopants such as magnesium. For example, an 8 keVimplant may be performed at a dose of 2×10¹³ cm⁻².

FIGS. 5A-5C are schematic plan views of gallium nitride based MMICdevices according to various embodiments of the present invention.

Referring first to FIG. 5A, a MMIC digital power amplifier 700 isillustrated that includes an amplifier stage 710 that is formed usingunit cell RF depletion mode transistors, a digital control block 720that is formed using a combination of digital depletion mode and digitalenhancement mode transistors, and a digital driver circuitry 730 that isformed using a combination of digital depletion mode and digitalenhancement mode transistors. The MMIC digital power amplifier 700 mayoperate as a switching amplifier in which the amplification transistorsthereof spend little time in the linear region (i.e., the transistorsare either fully on or fully off), which may allow the power amplifier700 to achieve very high efficiency levels (e.g., greater than 90%).

FIG. 5B is a schematic plan view of an integrated circuit 800 includingan RF transistor 810 and control circuitry 820 that includes bothenhancement mode and depletion mode transistors. The control circuitry820 modulates, limits or otherwise controls the bias of the RFtransistor 810 in order to affect the linearity, temperature response,gain response or other performance parameters of the RF transistor 810.

FIG. 5C is a schematic plan view of a MMIC transmit/receive circuit 900that is suitable for use in a time division duplex communication systemthat transmits and receives signals using the same frequency band. Asshown in FIG. 5C, the MMIC transmit/receive circuit 900 includes a highpower amplifier circuit 910, a low noise amplifier circuit 920, atransmit/receive switch 930 and a digital control block 940. The highpower amplifier circuit 910 and the low noise amplifier circuit 920 mayeach be implemented using unit cell RF depletion mode transistors. Thecontrol block 940 may include, for example, bias control circuits forthe high power amplifier 910 and/or for the low noise amplifier 920, andmay further include digital control circuits for controlling operationof the transmit/receive switch 930. The control block 940 may beimplemented, for example, using digital depletion mode and digitalenhancement mode transistors.

FIG. 6 is a flow chart illustrating a method of fabricating a galliumnitride based MMIC device according to certain embodiments of thepresent invention. As shown in FIG. 6, operations may begin with agallium nitride based channel layer being formed on a monolithicsubstrate (block 1000). The monolithic substrate may comprise, forexample, a silicon carbide wafer, and the gallium nitride based channellayer may comprise a single layer or multiple layers of gallium nitridebased materials. Next, a gallium nitride based barrier layer may beformed on the gallium nitride based channel layer opposite the substrate(block 1010). The gallium nitride based barrier layer may comprise asingle layer or multiple layers, and a bandgap of a lower portion of thegallium nitride based barrier layer is greater than a bandgap of anupper portion of the gallium nitride based channel layer that isimmediately adjacent the gallium nitride based barrier layer.

Next, an insulating layer is formed on the gallium nitride basedbarrier, the insulating layer including a plurality of first, second andthird gate electrode openings and a plurality of source/drain electrodeopenings that expose the gallium nitride based barrier layer (block1020). Then, a plurality of recesses are formed in the gallium nitridebased barrier layer in the first gate electrode openings (block 1030). Agate insulating layer is formed in the first gate electrode openings,the gate insulating layer covering sidewalls and bottom surfaces of therespective recesses (block 1040). The gate insulating layer may alsocover portions of the top surface of the insulating layer.

A plurality of first source electrodes, a plurality of second sourceelectrodes, a plurality of first drain electrodes and a plurality ofsecond drain electrodes are formed in the source/drain electrodeopenings in the insulating layer so that the first source electrodes,the second source electrodes, the first drain electrodes and the seconddrain electrodes directly contact a top surface of the gallium nitridebased barrier layer (block 1050). Then, an isolation implant process maybe performed (block 1060). First gate electrodes are formed in the firstgate electrode openings on the gate insulating layer, each first gateelectrode extending into a respective one of the recesses (block 1070).Second gate electrodes are formed in the second gate electrode openingsin the insulating layer, the second gate electrodes directly contactinga top surface of the gallium nitride based barrier layer (block 1080).Each set of a first source electrode, a first drain electrode and afirst gate electrode comprise the electrodes of an enhancement modetransistor, and each set of a second source electrode, a second drainelectrode and a second gate electrode comprise the electrodes of adepletion mode transistor.

It will be appreciated that the digital enhancement mode transistors(and the digital depletion mode transistors) may be used to implement avariety of different circuits, such as digital control circuitry,digital logic, and digital RF drivers.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present invention. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

As used herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”“comprising,” “includes” and/or “including” when used herein, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present. Itwill also be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “lateral” or “vertical” may be used herein to describe arelationship of one element, layer or region to another element, layeror region as illustrated in the figures. It will be understood thatthese terms are intended to encompass different orientations of thedevice in addition to the orientation depicted in the figures.

Embodiments of the invention are described herein with reference tocross-section illustrations that are schematic illustrations ofidealized embodiments (and intermediate structures) of the invention.The thickness of layers and regions in the drawings may be exaggeratedfor clarity. Additionally, variations from the shapes of theillustrations as a result, for example, of manufacturing techniquesand/or tolerances, are to be expected. Thus, embodiments of theinvention should not be construed as limited to the particular shapes ofregions illustrated herein but are to include deviations in shapes thatresult, for example, from manufacturing.

In the drawings and specification, there have been disclosed typicalembodiments of the invention and, although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation, the scope of the invention being set forth inthe following claims.

What is claimed is:
 1. A monolithic microwave integrated circuit,comprising: a monolithic substrate; a gallium nitride based channellayer on the monolithic substrate; a gallium nitride based barrier layeron the gallium nitride based channel layer opposite the monolithicsubstrate, the gallium nitride based barrier layer including a recessthat extends completely through the gallium nitride based barrier layerto expose the gallium nitride based channel layer; a depletion modetransistor having a first gate electrode that has a bottom surface thatis in direct contact with the gallium nitride based barrier layer; aninsulating layer in the recess; and an enhancement mode transistorhaving a second gate electrode that is on the insulating layer and thatextends into the recess.
 2. The monolithic microwave integrated circuitof claim 1, wherein the recess extends completely through the galliumnitride based barrier layer.
 3. The monolithic microwave integratedcircuit of claim 2, wherein the recess further extends into a topsurface of the gallium nitride based channel layer.
 4. The monolithicmicrowave integrated circuit of claim 1, wherein a first distancebetween the second gate electrode and a source electrode of theenhancement mode transistor is substantially the same as a seconddistance between the second gate electrode and a drain electrode of theenhancement mode transistor, and wherein a third distance between thefirst gate electrode and a source electrode of the depletion modetransistor is less than a fourth distance between the first gateelectrode and a drain electrode of the depletion mode transistor.
 5. Themonolithic microwave integrated circuit of claim 1, wherein thedepletion mode transistor includes a field plate and the enhancementmode transistor does not include a field plate.
 6. The monolithicmicrowave integrated circuit of claim 1, wherein the insulating layercomprises an oxide layer.
 7. The monolithic microwave integrated circuitof claim 1, wherein the depletion mode transistor has a first gate widththat exceeds a second gate width of the enhancement mode transistor byat least a factor of ten.
 8. A semiconductor integrated circuit,comprising: a monolithic substrate; a gallium nitride based barrierlayer on the monolithic substrate that includes a plurality of recessesin a top surface thereof, a radio frequency (RF) power amplifier formedon a first region of the monolithic substrate, the RF power amplifierincluding a plurality of gallium nitride based depletion modetransistors; and a digital circuit formed on a second region of themonolithic substrate, the digital circuit including a plurality ofgallium nitride based enhancement mode transistors, an insulating layerinterposed between the gallium nitride based barrier layer andrespective gate electrodes of the gallium nitride based enhancement modetransistors.
 9. The semiconductor integrated circuit of claim 8, whereinthe insulating layer comprises an oxide layer.
 10. The semiconductorintegrated circuit of claim 8, wherein the digital circuit furtherincludes a plurality of gallium nitride based depletion modetransistors.
 11. The semiconductor integrated circuit of claim 8,wherein the recesses extend completely through the gallium nitride basedbarrier layer.
 12. The semiconductor integrated circuit of claim 8,wherein each gallium nitride based enhancement mode transistor furtherincludes a source electrode and a drain electrode, and wherein the gateelectrode of each gallium nitride based enhancement mode transistor isequidistant between its corresponding source and drain electrodes. 13.The semiconductor integrated circuit of claim 12, wherein each galliumnitride based depletion mode transistor includes a gate electrode, asource electrode and a drain electrode, and wherein the gate electrodeof each gallium nitride based depletion mode transistor is closer to itscorresponding source electrode than it is to its corresponding drainelectrode.
 14. The semiconductor integrated circuit of claim 8, whereinthe gallium nitride based barrier layer is doped with first conductivitytype dopants under source and drain electrodes of each gallium nitridebased enhancement mode transistor, and the gallium nitride based barrierlayer is doped with second conductivity type dopants under the gateelectrodes of each gallium nitride based enhancement mode transistor,wherein the first conductivity type is opposite the second conductivitytype.
 15. A semiconductor integrated circuit, comprising: an epitaxialstructure that includes a plurality of recesses in a top surfacethereof, a radio frequency (RF) power amplifier in a first region of theepitaxial structure, the RF power amplifier including a plurality ofgallium nitride based depletion mode transistors; and a digital circuitin a second region of the epitaxial structure, the digital circuitincluding a plurality of gallium nitride based enhancement modetransistors and a plurality of gallium nitride based depletion modetransistors, wherein gate electrodes of the gallium nitride basedenhancement mode transistors in the second region of the epitaxialstructure directly contact sidewalls and bottom surfaces of therespective recesses.
 16. The semiconductor integrated circuit of claim15, wherein the gate electrodes of the gallium nitride based enhancementmode transistors are closer to the a bottom surface of the epitaxialstructure than are gate electrodes of the gallium nitride baseddepletion mode transistors that are in the digital circuit.
 17. Thesemiconductor integrated circuit of claim 16, wherein the gateelectrodes of the gallium nitride based enhancement mode transistors arecloser to the a bottom surface of the epitaxial structure than are gateelectrodes of the gallium nitride based depletion mode transistors thatare in the RFR Power amplifier.
 18. The semiconductor integrated circuitof claim 15, wherein each gallium nitride based enhancement modetransistor further includes a source electrode and a drain electrode,and wherein the gate electrode of each gallium nitride based enhancementmode transistor is equidistant between its corresponding source anddrain electrodes.
 19. The semiconductor integrated circuit of claim 18,wherein each gallium nitride based depletion mode transistor in thedigital circuit includes a gate electrode, a source electrode and adrain electrode, and wherein the gate electrode of each gallium nitridebased depletion mode transistor in the digital circuit is closer to itscorresponding source electrode than it is to its corresponding drainelectrode.
 20. The semiconductor integrated circuit of claim 15, whereinthe gallium nitride based depletion mode transistors in the RF amplifierhave a first gate width that exceeds a second gate width of the galliumnitride based depletion mode transistors in the digital circuit by atleast a factor of ten.